The Okelobondo natural fission reactor, southeast Gabon: Geology, mineralogy, and retardation of nuclear-reaction products

Nuclear fission reactors occur in the Oklo-Okelobondo uranium deposit in so utheast Gabon, The Okelobondo reactor zone (RZOKE, similar to 310 m depth) was the last reactor zone to be excavated before mining was terminated in D ecember 1997. RZOKE is situated at the base of a 2.5 m deep and 2.7 m wide reactor synform located between the FA sandstone and hydrothermally altered black shales-argile de pile-of the FB Formation. The reactor synform devel oped by hydrothermal dissolution of the FA sandstone during criticality at ca, 2 Ga. The core of RZOKE is up to similar to 55 cm thick and contains le ss than or equal to 90 vol% uraninite (less than or equal to 91.24 +/- 0.91 wt% UO2 and less than or equal to7.22 +/- 0.53 wt% PbO) embedded in a matr ix of Si-rich illite (6.08 to 7.91 Si per formula unit [p.f.u.]), The argil e de pile (typically less than or equal to 60 cm) consists mainly of donbas sitic chlorite (7.84 to 8.07 octahedrally coordinated Al p.f.u.) cut by fin e illite veinlets, Fe-rich chlorite (0.708 to 1.427 Fe p.f.u.) is abundant at the edges of the reactor zone. Organic matter in both the reactor zone a nd near-field surroundings played an important role in the enhancement of p orosity and stabilization of U during formation and operation of RZOKE, Fis siogenic Zr, Ce, Nd, and Th (daughter of Pu-240 and U-236 generated by neut ron-capture reactions) are well retained in uraninite and (U,Zr)-silicate. The (U,Zr)silicate formed during local migration of Zr, Sr, U, Pu, and lant hanide fission products during reactor criticality. Fissiogenic Ru, includi ng Ru-99-a daughter of Tc-99, was mainly retained as ruthenium arsenide (+/ -Pb, Co, Ni, and S). Although RZOKE generally appears well preserved, parti ally dissolved galena and uraninite, as well as the presence of accessory a nglesite(?) and leaduranyl sulfate hydroxide hydrate, suggest that recently observed deep, oxidized groundwaters have begun to interact with RZOKE.